In recent years, the communications industry has shown a growing interest in various types of wireless communications systems for communicating voice and/or data between numerous remote sites and a central location. It is well recognized that the use of a dedicated telephone facility for a conventional telephone system is not a convenient or economical option for all communications applications. For example, individuals who must be away from their base of operations have a need to conveniently and efficiently communicate with their base. Likewise, for many industrial applications, a central data collection site has a need for acquiring information from a variety of remotely located monitoring devices that collect data about the operation or performance of equipment. To overcome the limitations of a conventional telephone system, a two-way wireless communications link is often necessary to permit a response to a communication initiated from another location. In an attempt to solve the problem of supplying a response to an initial communication, the industry has offered various wireless communications systems, such as mobile radiotelephones.
One form of two-way communications is a cellular mobile radiotelephone (CMR) system, which is connected to the extensive public switched telephone network (PSTN) and permits communications between a mobile radiotelephone user and anyone with a conventional telephone (or another radiotelephone). Typical CMR systems are characterized by dividing a radio coverage area into smaller coverage areas or "cells" using low power transmitters and coverage-restricted receivers. As shown in U.S. Pat. Nos. 3,906,166 and 4,268,722, the limited coverage area enables the radio channels used in one cell to be reused in another cell. As a cellular mobile radiotelephone within one cell moves across the boundary of the cell and into an adjacent cell, control circuitry associated with the cells detects that the signal strength of the radiotelephone in the just-entered cell is stronger, and communications with the radiotelephone are "handed-off" to the just-entered cell. Thus, a CMR system can supply two-way communications for an array of cells, thereby supplying communications for a much wider area than conventional two-way radios.
A radiotelephone can "roam" between CMR systems and still place calls as well as remain accessible to receive calls from other callers. This is accomplished by means of a roaming system. Usually, a radiotelephone has a "home" CMR system in which the radiotelephone's Mobile Identification Number (MIN) and billing account are initially established. Because many CMR systems are capable of communicating information between themselves, the roaming system permits a home CMR system to contact a roaming radiotelephone by sharing information with a "foreign" CMR system. A CMR system can contact a radiotelephone and inform it about an incoming call by transmitting a Mobile Station Control Message to the radiotelephone within the cell in which the radiotelephone resides.
The most common of these Mobile Station Control Messages is commonly referred to as a "page", which is carried within a Mobile Station Control Message over a cellular network control channel. By monitoring the cellular network control channel, a radiotelephone within the CMR system can receive and analyze each page that is transmitted by the CMR system. Among other information, a page includes the MIN of the radiotelephone to which a connection is sought. The radiotelephone responds to a page that contains the MIN assigned to the radiotelephone. To avoid paging radiotelephones in every CMR system when a call is attempted, many CMR systems keep track of which radiotelephones are in the CMR system's coverage area.
Generally, a radiotelephone will monitor a cellular network control channel which transmits Mobile Station Control Messages including information such as the CMR system's System Parameter Overhead Message (SPOM) and the Global Action Overhead Message (GAOM). When the radiotelephone roams from its home CMR system and into a foreign CMR system, it will lose contact with the home CMR system's cellular network control channel, which is referred to as "losing synch." When a radiotelephone loses synch, it will scan the range of available radiotelephone frequencies for a new cellular network control channel. Where the radiotelephone has roamed into the coverage area of a foreign CMR system, the new cellular network control channel will be transmitting a control message that identifies the foreign CMR system. One way the control message can be used to distinguish the transmitting CMR system from other CMR systems is by including a unique System Identification Number (SID) with the control message's data. The monitoring radiotelephone will then recognize that the SID contained in the current control message is different from the SID of the control message that was previously received and will transmit an autonomous registration signal.
The autonomous registration signal tells the foreign CMR system that the radiotelephone is roaming and identifies the radiotelephone's home system. After contacting the radiotelephone's home CMR system and verifying that the roaming radiotelephone is a valid user of the home CMR system, the foreign CMR system will enable the radiotelephone to operate in the foreign CMR system as if the radiotelephone were in its home CMR system. Thus, the roaming radiotelephone can place and receive calls. Because the radiotelephone is registered as a user in the foreign CMR system, pages corresponding to calls being made to the radiotelephone need only be broadcast in the CMR system where the radiotelephone is registered.
A conventional CMR system operates on a 50 MHz range of frequencies in the 800 MHz and 2,000 MHz frequency bands. The frequency range is divided into channels which can transmit either control data or voice data. The control data is used to setup incoming and outgoing radiotelephone calls. The channels which carry such control data are the cellular network control channels, which are also described as "control channels."When a radiotelephone user attempts to place a call, the radiotelephone transmits control data over the control channel to the CMR system. This control data will be transmitted in the form of a "call origination signal" which alerts the CMR system to the user's desire to place a call. The CMR system responds by transmitting control data to the radiotelephone via the control channel to assign another channel (a voice channel) that can be used to transmit voice data (i.e., the callers' conversation). A conventional CMR system typically has 42 control channels that are divided into two sets of 21 control channels. Separate radiotelephone carriers (designated as Carrier A or Carrier B) can provide radiotelephone service support for the individual sets of control channels. For the purposes of this discussion, a radiotelephone will be referred to as being tuned to either "System A" or "System B", depending on the set of control channels that the radiotelephone is using to communicate.
A conventional CMR system has both cellular network control channels and cellular network voice channels. Generally, the control channels permit the transmission of the information used by the CMR system to complete a call placed by a radiotelephone or placed to a radiotelephone. The voice channels, on the other hand, are used to transmit the voice communications of the calling party and the called party, as well as a limited occasional amount of data related to the established voice communications (e.g., a power level change order). A conventional CMR system utilizes the control channels for a relatively short time to complete a call, as compared to the time that the CMR system must utilize the voice channels to permit a conversation between the parties.
Cellular network control channels are typically divided into two varieties. For communications originating with the CMR system's cell and directed toward a radiotelephone (i.e., in a forward direction), a Forward Control Channel (FOCC) is used. For communications from a radiotelephone toward the CMR system's cell (i.e., in the reverse direction), the Reverse Control Channel (RECC) is used. When a radiotelephone places a call, it sends a signal over the RECC to establish a connection with the cell so that the call can be placed and a voice channel can be allocated to enable conversation between the calling parties. On the other hand, when another party seeks to place a call to a radiotelephone user, the cell receives the incoming call request and transmits a page over the FOCC that notifies the radiotelephone that a connection is sought by the incoming call.
U.S. Pat. No. 5,546,444 describes a system for obtaining data and communicating data (rather than voices) over the control channels of a CMR system. Utilizing the control channels to communicate data conserves the valuable frequency spectrum allocated for the voice channels that support normal telephone conversations on the CMR system. Because the system can be implemented within the confines of a conventional CMR system, no significant modification is required to accommodate the ability to communicate data over the control channels.
An example of the use of a control channel to communicate data is provided by the use of a cellular device to monitor electricity usage. By connecting a modified radiotelephone transceiver to an electrical meter, data generated by that meter can be collected by a monitoring device and communicated, via the control channel, to a central data collection system by the transceiver. An interface between the monitor and the transceiver can convert the data to a format that is communicable by the transceiver. This combination, or monolithic integration, of a transceiver, monitor, and interface operates as a data reporting device. The central data collection system can cause to be placed, a page, via the FOCC, to the data reporting device in order to trigger a communication of the collected data. The data reporting device can respond by transmitting a message (i.e., an autonomous registration message or a call origination message), via the RECC, for delivery to the central data collection system. Instead of sending data for completing the registration or origination procedure, however, the data reporting device can replace such data with the collected data. Thus, no voice channel connection is established, but the collected data is, nonetheless, communicated to the central data collection system.
Another example is that of the long-haul truck. Long-haul trucking companies have a desire to monitor various aspects of trucks that are in transit and are scattered all over the country. By placing a mobile data reporting device on the truck, trailer, or container, this data can be collected and communicated back to a central data collection system. For example, a truck may be equipped with a Global Positioning System sensor that generates the coordinates of the truck's location. By sending that data back to the data collection system, the long-haul trucking company could keep track of the truck's movements at all times.
Similarly, the data could include the current temperature of a refrigerated compartment, the number of containers aboard the truck, or the speed of travel. However, unlike the electrical meter data reporting device, the mobile data reporting device may travel between CMR system cells and between CMR systems.
Thus, the data collection system will need to be told in which CMR system the long-haul truck resides, so that the data collection system can communicate with the mobile data reporting device in the forward direction, via the FOCC.
This mobile data reporting device can communicate with the data collection system in the reverse direction, as can any radiotelephone in a "foreign" CMR system, via the RECC. The data collection system described in U.S. Pat. No. 5,546,444 does not readily accommodate mobile data reporting devices, in that it does not inform the data collection system as to the location of a mobile data reporting device that has left a particular CMR system. In order for the data collection system to communicate in the forward direction to a mobile data reporting device in a foreign CMR system, it would have to send out a page in multiple CMR systems. This is an expensive and ineffectual means of establishing such communications, because it would result in an overuse of the forward direction control channels of the CMR systems' cells.
Typically, CMR systems require a radiotelephone to transmit an "autonomous registration signal" when the radiotelephone enters a foreign CMR system from another CMR system. A radiotelephone typically determines whether it has entered the coverage area of a foreign CMR system by comparing a SID assigned to the radiotelephone's home CMR system to the SID most recently received. Conventional CMR systems transmit a control message containing a SID over the FOCC. If the assigned SID is different from the SID most recently received, then the radiotelephone determines that it has moved into a foreign CMR system. However, some CMR systems do not transmit a SID different than that generated by adjacent CMR systems. In such a case, the radiotelephone is cannot determine that it needs to transmit the autonomous registration signal and roaming is not enabled. A radiotelephone that is within the coverage area of such a CMR system is "lost" to the radiotelephone's home CMR system and cannot receive incoming calls without flooding the FOCCs of multiple CMR systems with a page to initiate contact with the radiotelephone.
For the remote data reporting device, such as the electrical meter monitoring example, this does not present a problem, because the monitor never changes location, and the "home" CMR system knows where the remote data reporting device can be located, should communication in the forward direction be required. However, many applications of such data collection systems require the ability to communicate in the forward direction (i.e., from the data collection system to the data reporting device). To do so, the central data collection entity must know in which CMR system the data reporting device resides. Without this information, the central data collection entity would have to request a page in all CMR systems in order to locate the data collection device, which would result in an overuse of the forward direction control channels of the CMR systems.
There is another problem with collecting data from mobile data reporting devices that the data collection system described in U.S. Pat. No. 5,546,444 does not accommodate. The problem occurs because not every CMR system is equipped to support a data collection system. Where a data collection system incorporates a mobile data reporting device, the mobile data reporting device may, on occasion, roam into a CMR system's coverage area where the CMR system is not equipped to cooperate with the data collection system. Until the mobile data reporting device roams into a CMR system that is so equipped, it will be lost to the home CMR system and data communication will be impossible.
Accordingly, there is a need to overcome the limitations of the prior art by adapting an existing communications network to communicate data between a central location and numerous mobile data reporting devices in both forward and reverse directions. There is also a need to configure the mobile data reporting devices such that they are capable of communicating data to the central data collection system despite being within the coverage area where one of the CMR systems is not equipped to accommodate data communication. This new use of an existing communications system should have a minimum impact upon present communications carried by the CMR system. The present invention adapts the existing architecture of a CMR system in an efficient and cost-effective manner to support data communications via the CMR system, including the collection and reporting of data recorded by mobile data reporting devices that travel between CMR systems.